High fidelity sound reproduction using a cone-type loudspeaker, is hampered by interference between the sound vibrations set up at the front of the cone and those set up at the rear. If the vibrations at the rear are allowed to travel unrestricted to the front, constructive and destructive acoustic interference at different frequencies will seriously distort the frequency response. Previous workers have mounted speakers in boxes with the front of the cone facing the room and the interior of the box containing sound-absorbing material. The problem of interference from the rear of the speaker was thus solved by absorbing approximately half the acoustic energy generated by the speaker. A further solution, called an airsuspension speaker enclosure, provided the speaker sealed to one face of an air-tight box. Again, approximately half the acoustic energy was wasted in the box.
A further problem with loudspeakers, particularly at low frequencies, is obtaining adequate acoustic coupling between the speaker cone and the air. One solution has been to use a very large speaker cone, for example 24 inches and larger. Another solution has been to make the speaker cone very compliant thus allowing very large physical excursions and thereby moving a large quantity of air. Although these solutions improve the low-frequency performance, they degrade the mid- and high-frequency performance sufficiently that three, four and more speakers, each fed a particular range of frequencies from an electric network, are required to reproduce the useful acoustic spectrum of from about 30 to about 20,000 hertz. Another way of achieving acoustic coupling employs an air column inside a divergent horn. In theory, each acoustic frequency is able to find a cross-sectional area of the horn at which an acoustic impedance match with the air is achieved.
One approach to solving both the back-to-front interference problem and the low-frequency coupling problem has been called the tuned-port speaker enclosure. In a tuned-port speaker enclosure, the front of the speaker faces the room, with or without a horn-type device in the path, while the rear of the speaker faces a passage of considerable length, usually folded to reduce its physical dimensions, which terminates in a port usually adjacent to and facing in the same direction as the front of the speaker. The dimensions and length of the passage delays the emergence of the acoustic energy from the rear of the speaker enough that interference is avoided. Furthermore, the horn-like nature of the passage can be made to improve the acoustic coupling of certain frequencies between the speaker and the air. Tuned-port enclosures require sturdy, relatively large and massive structures to contain the acoustic pressures generated in them without adding their own vibrational distortion to the emerging sound.